3D printing of dynamic covalent polymer network with on-demand geometric and mechanical reprogrammability

Delicate geometries and suitable mechanical properties are essential for device applications of polymer materials. 3D printing offers unprecedented versatility, but the geometries and mechanical properties are typically fixed after printing. Here, we report a 3D photo-printable dynamic covalent network that can undergo two independently controllable bond exchange reactions, allowing reprogramming the geometry and mechanical properties after printing. Specifically, the network is designed to contain hindered urea bonds and pendant hydroxyl groups. The homolytic exchange between hindered urea bonds allows reconfiguring the printed shape without affecting the network topology and mechanical properties. Under different conditions, the hindered urea bonds are transformed into urethane bonds via exchange reactions with hydroxyl groups, which permits tailoring of the mechanical properties. The freedom to reprogram the shape and properties in an on-demand fashion offers the opportunity to produce multiple 3D printed products from one single printing step.


Additional experimental descriptions
Synthesis of hindered urea containing bismethacrylate (HUBM). HUBM was synthesized as follows. 2-(Tert-butylamino)ethyl methacrylate (3.7 g, 0.02 mol) was added into a three-necked flask with a set temperature of 50 °C, followed by the dropwise addition of a stoichiometric amount of hexamethylene diisocyanate (1.68 g, 0.01 mol). After reaction under magnetic stirring for 5 hours, the product as a viscous liquid was obtained without further purification (yield: 99%). The structure was confirmed by 1  Model compound experiment of n-hexyl-n-tert-butylethyl-urea and 3-methyl-1butanol. A monomer containing monofunctional hindered urea n-hexyl-n-tertbutylethyl-urea was obtained with the addition of an equimolar of n-tertbutylethylamine (1.01 g, 0.01 mol) and hexyl isocyanate (1.27 g, 0.01 mol) following the same procedure for HUBM (50 °C for 5 hours). The product as a liquid was obtained without further purification (yield: 99%). The structure was confirmed by 1  In the model compound experiment, a stoichiometric amount of n-hexyl-n-tertbutylethyl-urea (2.28 g, 0.01 mol) and 3-methyl-1-butanol (0.88 g, 0.01 mol) were mixed and allowed to react at 120 °C. The reaction conversion αm was quantified by 1 H NMR analysis. Specifically, the αm value was calculated as the ratio between the integrated area of peak a′ (3.7 ppm) and the total integrated area of a (3.9 ppm) and a′ with all the peaks normalized using b (1.7 ppm) as the internal standard. All the results were normalized for calculation.
Model compound experiment of TBEMA and 3-methyl-1-butanol. A stoichiometric amount of 2-(tert-butylamino)ethyl methacrylate (1.85 g, 0.01 mol) and 3-methyl-1butanol (0.88 g, 0.01 mol) were mixed and allowed to react at 120 °C. The reaction conversion α1 was quantified by 1 H NMR analysis. Specifically, the α1 value was calculated as the ratio between the integrated area of peak a′ (2.75 ppm) and the total integrated area of a (2.87 ppm) and a′ with all the peaks normalized using b (6.1 ppm) as the internal standard. All the results were normalized for calculation. In the model compound experiment, a stoichiometric amount of (2-(tert-butyl)-hexylurea)ethyl methacrylate (3.12 g, 0.01 mol) and 3-methyl-1-butanol (0.88 g, 0.01 mol)were mixed and allowed to react at 120 °C. The reaction conversion α2 was quantified by 1 H NMR analysis. Specifically, the α2 value was calculated as the ratio between the integrated area of peak a′ (4.27 ppm) and the total integrated area of a (4.2 ppm) and a′ with all the peaks normalized using b (6.1 ppm) as the internal standard.

Measurement of hindered urea bond conversion in polymer networks. The samples
were annealed at a given temperature (80 °C or 120 °C) for different durations. The conversion of the hindered urea was measured by Fourier transform infrared (FTIR) spectrometer (Nicolet 5700 infrared spectrometer) by monitoring the integrated peak area of hindered urea group (Ahindered urea, 1635 cm −1 ) and urethane group (Aurethane, 1725 cm −1 ). The conversion αn was calculated as follows: (1) All the results were normalized for calculation.

Swelling properties and the calculations of the average molecular weight between crosslinks
Mc. The swelling experiments were conducted by immersing the samples in tetrahydrofuran (THF) and a minimum of three specimens were tested for each sample.
The swelling degree Q is calculated from the following equation: where ms and m0 are corresponding to the weight of the swollen sample and dried sample.
According to the Flory-Rehner equation 1 , the average molecular weight between crosslinks Mc can be calculated as follows: where VP and Vl are corresponding to the specific volume of the polymer and molar Supplementary Figure 6 FTIR spectrum of n-hexyl-n-tert-butylethyl-urea. The peak at 2264 cm -1 that corresponds to the stretching vibration of the isocyanate group completely disappears, indicating the successful synthesis of target products. The peak at 1635 cm -1 corresponds to the formation of the hindered urea bond.